Most metals are obtained by removing those metal values from the ores in which they are found in the ground. Once the ore has been mined, the metal must then be separated from the remainder of the ore. One method to separate the metal from the ore is known as leaching. In general, the first step in this process is contacting the mined ore with an aqueous is solution containing a leaching agent which extracts the metal from the ore into solution. For example, in copper leaching operations, such as, for example, in the agitation leaching of copper oxide ores, sulfuric acid in an aqueous solution is contacted with copper oxide minerals. During the leaching process, acid in the leach solution is consumed and copper is dissolved thereby increasing the copper content of the aqueous solution.
The aqueous leach solution containing the leached metal can then be treated via a known process referred to as solvent extraction wherein the aqueous leach solution is contacted with a nonaqueous solution containing a metal-specific extraction reagent. The metal-specific extraction reagent extracts the metal from the aqueous phase into the non-aqueous phase. During the solvent extraction process for copper and certain other metals, the leaching agent is regenerated in the aqueous phase. In the case where sulfuric acid is the leaching agent, sulfuric acid is regenerated in the aqueous phase when copper is extracted into the organic phase by the extraction reagent. Normally, for every ton of copper removed from the leach solution about 1.5 tons of sulfuric acid is generated in the leach solution.
Leaching agents are often recycled back to the leaching process to dissolve more metal and the more leaching agent that can be recycled the less that needs to be obtained from another source. In a standard agitation leaching process for copper, followed by solvent extraction, the leach solution is diluted to a lesser or greater extent with water in conjunction with the solid-liquid separation process needed to provide a clarified leach liquor and tailings. The diluted clarified leach solution is then transferred to one or more solvent extraction plants depending on the volume of leach solution and the capacity of each plant. The diluted leach solution undergoes solvent extraction wherein copper is removed from, and the sulfuric acid concentration is increased in, the aqueous phase. A portion of this copper-depleted, acid-containing aqueous phase, now called the raffinate, is then recycled back to the leaching process. The other portion is recycled back to the front of the solid-liquid separation process where it dilutes the leach solution exiting the agitation leaching process. Depending on the acid balance across the whole process some of this recycled aqueous phase may be partially neutralized.
The leach solution from an agitation leach process is normally diluted during the solid-liquid separation step in order to maximize the washing of the leached solids so that metal lost to the solids is minimized. During solvent extraction as the metal is extracted, acid concentration builds in the aqueous phase and the reaction becomes self-limiting in equilibrium. However, because of the initial dilution to maximize metal recovery from the leached solids, the amount of acid regenerated is lower in concentration than it would have been if the leach solution had not been diluted in the washing of the leached solids. Unfortunately, the lower the concentration of acid in the recycled raffinate, the more fresh acid that needs to be added and this increases the cost of the operation.
Accordingly, there is a need in the art for improved processes for metal leaching and solvent extraction, wherein the recovery of leaching agents is improved without negatively affecting metal recovery.